1. Field of the Invention
The present invention relate to a display device and, more particularly, to a liquid crystal display (LCD) device and a fabrication method thereof. Although embodiments of the invention are suitable for a wide scope of applications, it is particularly suitable for simplifying a fabrication process, reducing fabrication costs and improving a production yield by reducing the number of masks.
2. Discussion of the Related Art
As consumers' interest in information displays grows and the demand for portable (mobile) information devices increases, research and commercialization of light and thin flat panel displays (“FPD”) has increased.
Among FPDs, the liquid crystal display (“LCD”) is a device for displaying images using optical anisotropy of liquid crystal. LCD devices exhibit excellent resolution and color and picture quality. Thus, LCD devices can be widely used for notebook computers or desktop monitors, and the like.
The LCD device includes a color filter substrate, an array substrate and a liquid crystal layer formed between the color filter substrate and the array substrate.
An active matrix (AM) driving method commonly used for the LCD is a method in which liquid crystal molecules in a pixel part are driven by using amorphous silicon thin film transistors (a-Si TFTs) as switching elements.
In the fabricating process of the LCD, a plurality of masking processes (namely, photolithographic processes) are performed to fabricate the array substrate including the TFTs. A method for reducing the number of masking process will increase productivity.
FIG. 1 is an exploded perspective view showing a related art LCD device.
As shown in FIG. 1, the LCD device includes a color filter substrate 5, an array substrate 10 and a liquid crystal layer 30 formed between the color filter substrate 5 and the array substrate 10.
The color filter substrate 5 includes a color filter (C) including a plurality of sub-color filters 7 that implement red, green and blue colors, a black matrix 6 for dividing the sub-color filters 7 and blocking light transmission through the liquid crystal layer 30, and a transparent common electrode 8 for applying a voltage to the liquid crystal layer 30.
The array substrate 10 includes gate lines 16 and data lines 17 which are arranged vertically and horizontally to define a plurality of pixel areas (P), TFTs (T), switching elements, formed at respective crossings of the gate lines 16 and the data lines 17, and pixel electrodes 18 formed on the pixel areas (P).
The color filter substrate 5 and the array substrate 10 are attached in a facing manner by a sealant (not shown) formed at an edge of an image display region to form a liquid crystal panel, and the attachment of the color filter substrates 5 and the array substrate 10 is made by an attachment key (not shown) formed on the color filter substrate 5 or the array substrate 10.
FIGS. 2A to 2E are cross-sectional views sequentially showing a fabrication process of the array substrate of the LCD device in FIG. 1.
As shown in FIG. 2A, a gate electrode 21 made of a conductive material is formed by using a photolithography process (a first masking process) on a substrate.
Next, as shown in 2B, a first insulating layer 15a, an amorphous silicon thin film and an n+ amorphous silicon thin film are sequentially deposited over the entire surface of the substrate 10 with the gate electrode 21 formed thereon, and the amorphous silicon thin film and the n+ amorphous silicon thin film are selectively patterned by using the photolithography process (a second masking process) to form an active pattern 24 formed of the amorphous silicon thin film on the gate electrode 21.
In this case, the n+ amorphous silicon thin film pattern 25 which has been patterned in the same form as the active pattern 24 is formed on the active pattern 24.
Thereafter, as shown in FIG. 2C, an opaque conductive film is deposited over the entire surface of the array substrate 10 and then selectively patterned by using a photolithography process (a third masking process) to form a source electrode 22 and a drain electrode 23 at an upper portion of the active pattern 24. At this time, a certain portion of the n+ amorphous silicon thin film pattern formed on the active pattern 24 is removed through the third masking process to form an ohmic-contact layer 25′ between the active pattern 24 and the source and drain electrodes 22 and 23.
Subsequently, as shown in FIG. 2D, a second insulating layer 15b is deposited over the entire surface of the array substrate 10 with the source electrode 22 and the drain electrode 23 formed thereon, and a portion of the second insulating layer 15b is removed through a photolithography process (a fourth masking process) to form a contact hole 40 exposing a portion of the drain electrode 23.
Finally, as shown in FIG. 2E, a transparent conductive film is deposited over the entire surface of the array substrate 10 and then selectively patterned by using a photolithography process (a fifth making process) to form a pixel electrode 18 electrically connected with the drain electrode 23 via the contact hole 40.
As mentioned above, in fabricating the array substrate including TFTs, according to the related art, a total of five photolithography processes are necessarily performed to pattern the gate electrode, the active pattern, the source and drain electrodes, the contact hole and the pixel electrode.
The photolithography process is a process of transferring a pattern formed on a mask onto the substrate on which a thin film has been deposited to form a desired pattern, which includes a plurality of processes such as a process of coating a photosensitive solution, an exposing process and a developing process, etc, resulting in degradation of the production yield.
Generally, the masks designed for forming the patterns are quite expensive, so as the number of masks used in the processes increases, the fabrication cost of the LCD increases proportionally.